1. Field of the Invention
The present invention relates to methods and compositions for relatively high temperature superconductive materials. More particularly, an embodiment of the invention relates to at least partially covering a superconductive material by a coating material, the coating material providing resistance to corrosion and enhanced conductivity to the underlying superconductive material.
2. Brief Description of the Related Art
Superconductive materials have little or no resistance to a direct current. As defined herein the term "critical temperature" (T.sub.C) refers to the temperature at which a material has substantially no resistance to a direct current. A superconductor is said to exist in its normal state above T.sub.C and be in its superconductive state below T.sub.C ; T.sub.C decreases with increasing applied magnetic field (H). In the superconductive state below T.sub.C, a superconductor excludes a magnetic field and offers no resistance to a direct current. Type I superconductors exclude the magnetic field from the bulk of the material below a critical magnetic field (H.sub.C) that increases with decreasing temperature; however, when H&gt;H.sub.C the material reverts to its normal (i.e., resistive) state. Type II superconductors allow bundles of magnetic flux to penetrate into the bulk without loss of superconductivity in a range of magnetic fields H.sub.C1 &lt;H&lt;H.sub.C2 ; these bundles are referred to as vortices since they are confined by a superconductive vortex current. A type II superconductor remains superconductive below H.sub.C2 and offers no resistance to a direct current so long as the vortices remain stationary. A direct current has a magnetic field associated with it that increases linearly with the current. If this internal field exceeds H.sub.C (Type I) or H.sub.C2 (Type II), then the superconductor reverts to its normal (resistive) state. It follows that there is a critical current density J.sub.C beyond which the material loses its superconductive properties. For direct-current applications, J.sub.C is an important feature. Practical superconductors are Type II with a high H.sub.C2 at a desired operating temperature T.sub.OP &lt;T.sub.C ; Type I superconductors have an H.sub.C which is too low for practical uses. The high-T.sub.C copper-oxide superconductors are Type II with a high H.sub.C2, but their layered structure and high T.sub.C introduce additional problems. The internal magnetic field associated with a direct current exerts a force on the bundles of magnetic flux trapped within vortices; and if this force is strong enough to cause the vortices to move, the current experiences a resistance. It is desired to find a way to pin the vortices to a stationary configuration to prevent resistance, even if H&gt;H.sub.C2. Pinning tends to become more difficult as T.sub.C is raised.
The discovery of copper-oxide superconductors having a T.sub.C.gtoreq.90 K has introduced the possibility of operating superconductive materials in liquid nitrogen, which boils at 77 K. If a flexible, chemically stable wire or tape, with a J.sub.C.gtoreq.10.sup.7 A/cm.sup.2, can be achieved at a T.sub.OP =77 K, then superconductive coils may be used to generate high magnetic fields in liquid nitrogen with minimal resistive loss. Realization of this goal would open up a number of technical applications, including levitated trains, high-capacity electrical storage, low-loss power transmission, electric motors and generators, transformers, and high-field magnets. Unfortunately, the copper-oxides are brittle ceramics that have a large electrical anisotropy owing to their layered structure. This situation tends to restrict the development of magnetic coils to superconductive films deposited on a flexible substrate The films having a relatively high J.sub.C at 77 K tend to corrode on exposure to moist air, and at 77 K the vortices tend to become mobile (i.e., the liquid-vortex state) at a J.sub.C lower than is needed.
In order to improve the resistance of these materials to corrosion, a number of analogs of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. have been synthesized. The synthesis of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. analogs that are resistant to corrosion is described in U.S. Pat. No. 5,591,696 to McDevitt et al., which is incorporated by reference as if set forth herein. McDevitt et. al. shows that derivatives of YBa.sub.2 CU.sub.3 O.sub.7-.delta., in which a portion of the Y, Ba, and/or Cu atoms are replaced by a single rare-earth element (e.g., La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, or Y) and an alkaline-earth metal (Ca or Sr), show increased resistance to corrosion. The increase in stability in these systems is believed to arise from blocking access of protonic species to the interior of the lattice. The replacement of Y, Ba, and/or Cu by a rare-earth metal may block channels which are typically present in YBa.sub.2 Cu.sub.3 O.sub.7-.delta. materials. The blockage of these channels also tends to increase the stability of the corrosion resistant material's lattice. Moreover, changes in the lattice stress and strain factors which accompany the above mentioned substitutions can alter the reactivity characteristics of these technologically important materials, as can alterations in the electrostatic effects caused by the same metal ion replacements. These chemical isomerizations, however, produce compounds having a somewhat lower T.sub.C than the parent YBa.sub.2 Cu.sub.3 O.sub.7-.delta.. For example, a sample of Y.sub.0.6 Ca.sub.0.4 Ba.sub.1.6 La.sub.0.4 Cu.sub.3 O.sub.6.96, which exhibited very little corrosion after 2 months, has a T.sub.C of 80 K, as opposed to 92 K for YBa.sub.2 Cu.sub.3 O.sub.7-.delta..
McDevitt et al. further describe the use of corrosion resistant materials, such as Y.sub.0.6 Ca.sub.0.4 Ba.sub.1.6 La.sub.0.4 CU.sub.3 O.sub.6.96, to encapsulate a thin layer of YBa.sub.2 Cu.sub.3 O.sub.7-.delta.. The corrosion resistant material protects the more chemically reactive superconductive material YBa.sub.2 CU.sub.3 O.sub.7-.delta. from the atmosphere.
The embedded superconductive materials of McDevitt et al., however, do not address the problem in which high T.sub.C superconductive materials tend to lose their superconductivity properties when exposed to strong magnetic fields. Typically, the magnetic response of a type II superconductive material depends on the strength of the applied magnetic field and the temperature. Conventional type II superconductive materials (e.g., superconductive materials having a T.sub.C below about 25 K) tend to show three distinct magnetic states (H&lt;H.sub.C1) 10, (H.sub.C1 &lt;H&lt;H.sub.C2) 20 and (H&gt;H.sub.C2) 30, as depicted in FIG. 1. The first state 10 is called the Meissner state. In the Meissner state 10 the superconductive material expels any applied magnetic field. The superconductive material will tend to remain in this state as long as the applied magnetic field remains below a certain field strength (H.sub.C1) 15. This field strength (H.sub.C1) 15, called the lower critical field strength, is dependent on the temperature of the material.
The second magnetic state 20, the vortex solid state, emerges if an applied magnetic field increases to a value higher than the lower critical field strength (H.sub.C1) 15. At this point, the magnetic field can penetrate the superconductive material, but not completely or uniformly. Instead, discrete magnetic flux lines known as vortices are formed within the superconducting material. Each of these vortex lines includes flowing currents circulating around a non-superconducting core. When an applied current is passed through a type II superconducting material exposed to an applied magnetic field, it adds to the circulating currents on one side of the vortex and subtracts from the current on the other side. As a result of this action, a force acts on the vortex line. The force tends to make the vortex move in a direction at right angles to both the applied current and the vortex line. In conventional type II superconductive materials, the vortex lines are not significantly moved due to the current flow because of the low temperatures at which these materials exhibit superconductivity. The lines are said to be "frozen" within the superconductive material.
The third state 30 occurs if the applied magnetic field strength reaches a second higher critical point. At this second critical point the superconductive material behaves as a normal metal. This loss of superconductive qualities occurs because increases in the strength of the magnetic field force the vortex lines to move closer together. When the vortex cores, which are non-superconductive, overlap too much, there is no longer any room between the vortices to maintain superconductivity.
A problem with high T.sub.C type II superconductive materials is that they exhibit a fourth magnetic state, known as the vortex liquid state 40, as depicted in FIG. 2. As an applied magnetic field strength is increased, the vortex lines begin to appear. However, at the higher temperatures (e.g., above 25 K) typically used for high T.sub.C superconductive materials, the vortex lines begin to orient themselves in a non-rigid lattice. In effect, the vortices "melt" into a "liquid" state. This allows the vortex lines to move about the superconductive material when a current is applied in the presence of a magnetic field. This movement of the vortex lines tends to dissipate the current, and create resistance within the sample. Thus, high T.sub.C superconductive materials tend to lose their superconductivity as the strength of an applied magnetic field is increased.
It is therefore desirable to increase the chemical stability of copper-oxide superconductive films, while simultaneously increasing the strength of the vortex pinning.